Processing of Polytetrafluoroethylene Resins

Abstract

This chapter covers the processing of polytetrafluoroethylene (PTFE) resins, which include granular and fine powder resins. These materials are very different, therefore each requires different processing techniques. Granular resins are processed by a technique similar to that in powder metallurgy. They are commonly processed by compression molding and ram extrusion. In compression molding the resins are preformed into a billet, which is then sintered and cooled. Cooling and its rate are important because they have important effects on the final products. Billets are frequently converted to films and sheets by skiving. Other molding methods are automatic molding used for small parts, coining used for parts with complex shapes, and isostatic molding using uniform pressure from all directions. Ram extrusion is a process to produce PTFE extrudates of continuous lengths. The resin is fed into one end of a straight die tube of uniform diameter, where it is compacted by a ram and forced through the tube, which incorporates a heated sintering zone. Granular PTFE resins are most frequently extruded as rods or tubes, but it is possible to produce extrudates of noncircular cross-sections. The processing of fine powders is more complex. Because PTFE fine powders do not melt and flow, they are commonly fabricated by a technology adopted from ceramic processing called paste extrusion. In paste extrusion, the paste is prepared by mixing the powder with a 15 to 25% hydrocarbon lubricant. The basic steps are preparation of the extrusion mix, preforming, extrusion, and drying. Additional steps are used for specific products. Some extrudates are calendered or sintered, and stretched. Typical products are unsintered tape, wire and cable insulation, tubing, rods, and articles from expanded PTFE, such as membranes, sheets, and tubing.